Non-volatile semiconductor memory cells using a floating gate to store charges thereon and memory arrays of such non-volatile memory cells formed in a semiconductor substrate are well known in the art. Typically, such floating gate memory cells have been of the split gate type, or stacked gate type.
Read operations usually are performed on floating gate memory cells using sense amplifiers. A sense amplifier for this purpose is disclosed in U.S. Pat. No. 5,386,158 (the “'158 Patent”), which is incorporated herein by reference for all purposes. The '158 Patent discloses using a reference cell that draws a known amount of current. The '158 Patent relies upon a current mirror to mirror the current drawn by the reference cell, and another current mirror to mirror the current drawn by the selected memory cell. The current in each current mirror is then compared, and the value stored in the memory cell (e.g., 0 or 1) can be determined based on which current is greater.
Another sense amplifier is disclosed in U.S. Pat. No. 5,910,914 (the “'914 Patent”), which is incorporated herein by reference for all purposes. The '914 Patent discloses a sense circuit for a multi-level floating gate memory cell, which can store more than one bit of data. It discloses the use of multiple dummy cells that are utilized to determine the value stored in the memory cell (e.g., 00, 01, 10, or 11). Current mirrors are utilized in this approach as well.
One issue that arises in sense amplifiers is that the performance of the sense amplifier can be affected by transistor mismatching. Transistor mismatching can be alleviated by trimming or calibrating the sense amplifiers during testing. However, for example, when transistors heat up during operation, they can become mismatched again. This can cause components such as current mirrors to behave in an unintended manner, which will cause the sense amplifier to read data inaccurately. In the prior art, there has been no mechanism for correcting transistor mismatching during operation of the sense amplifier.
What is needed is a mechanism for reducing transistor mismatching during actual operation of a sense amplifier.